1. Field of the Invention:
The present invention relates generally to an improvement in a cathode ray tube apparatus and more particularly to a cathode ray tube apparatus wherein high resolution is obtainable on all the parts of the phosphor screen.
2. Description of the Prior Art:
Generally, resolution of a cathode ray tube apparatus depends on the size and shape of beam spots produced on phosphor screen, and it is important to obtain beam spots having as small size and distortion as possible in order to obtain the high resolution. Furthermore, in a color cathode ray tube apparatus an important factor for the high resolution is that three beam spots made by three electron beams are concentrated on same position on the phosphor screen. Accordingly, in a color cathode ray tube apparatus of in-line type, magnetic field of the horizontal deflection member is designed to have a pin-cushion shaped distribution of magnetic flux as shown in FIG. 1(a) and magnetic field of a vertical deflection member has a barrel shape distribution of magnetic flux as shown in FIG. 1(b), thereby to achieve self-convergence of three electron beams 1, 2 and 3.
However, the above-mentioned way of the self-convergence has a problem that,though the convergence of three electron beams is improved, cross sections of three electron beams become distorted as the beam deflection angles increase. Therefore the beam spots produced at corner areas on the phosphor screen is liable to have distortions as shown in FIG. 2. That is, though the beam spot 5 produced at the center part of the phosphor screen 4 becomes circular, the beam spots 6 produced at the corner parts and peripheral parts are formed in a shape to include vertically oblong low brightness haze part 8, resulting in difficulty of achieving high resolution at the peripheral parts of the phosphor screen.
The above-mentioned shape distortion of the beam spots are induced by the application of non-uniform magnetic field of the deflection member as shown in FIG. 1(a) and FIG. 1(b) to the three electron beam set of the self-convergence type cathode ray tube apparatus, and deflection aberration of electron beams in the deflection magnetic field is produced as a result of strengthening of focusing in vertical direction.